The present invention relates, to an optical pick-up device and an objective lens used therein, for example, to an optical pick-up device which can perform information recording and/or information reproduction for two optical information recording media each having a different thickness of a transparent base board and an objective lens used therein.
With the practical use of a short wave red semiconductor laser in recent years, there has been developed a high density optical disk DVD (digital video disk) that is the same in size as a conventional optical disk CD (compact disk) representing an information recording medium, and has a larger capacity. In order to attain high density of a recording signal, it is requested to make a spot formed on a recording medium by an objective lens to be smaller in an optical system of an optical information recording/reproducing device employing such an optical disk. Therefore, there are the facts that a wavelength of a laser representing a light source is made to be shorter and a numerical aperture of an objective lens is made to be higher.
For example, in an optical pick-up device that performs information recording and/or reproduction for DVD, when a short wave semiconductor laser ray having a wavelength of about 650 nm is used as a light source, numerical aperture NA closer to the optical disk is approximately 0.6 for the objective lens that converges the laser ray. Incidentally, in CD and DVD, there are optical disks under various kinds of standards, including, for example, CD-R (write-once read-many compact disk), and in addition to CD and DVD, there is MD (mini disk) that has been commercialized and has become popular.
On the other hand, in the optical pick-up device that performs information recording and/or reproduction for CD-R, it is necessary to use the light source having a wavelength of about 780 nm. For the other optical disks, it is possible to use the light source having a wavelength other than the specific wavelength. In this case, a necessary numerical aperture NA can be changed in accordance with the wavelength xcex of the light source used therein. For example, the necessary numerical aperture is NA=xcex (xcexcm)/1.73 approximately for CD, while the necessary numerical aperture is NA=xcex (xcexcm)/1.06 approximately for DVD. The numerical aperture in the present specification means a numerical aperture of the converging optical system viewed from the optical disk side. The necessary numerical aperture means a numerical aperture that is calculated by both spot size d required on the recording surface of the optical disk and wavelength xcex in use, and that is commonly shown by the formula NA=0.83xc3x97xcex/d.
As mentioned above, in the market, there exist various kinds of the optical disks having different transparent base board thicknesses, different recording densities and different wavelengths in use. However it is too heavy a burden for the user to purchase the exclusive information recording and reproduction device which can perform information recording and/or reproduction for each optical disk. Therefore, there is proposed the information recording/reproduction device provided with the interchangeable optical pick-up device that can cope with the various optical disks.
In such an optical disk, when light fluxes each having a different wavelength are made to enter the base boards each having a different thickness, it is necessary to correct spherical aberration to be the prescribed value or less, and to control a spot diameter of each light flux to be within the prescribed range, in order to perform information writing and reading properly.
As a solution for the foregoing, there is proposed the optical pick-up device wherein converging optical systems are provided for different optical disks respectively, and optical converging systems are switched in accordance with the optical disks to be reproduced. The optical pick-up device mentioned above makes it possible to correct the spherical aberration to be under the prescribed value, and to make the spot diameter of each light flux to be within the prescribed range, when the light fluxes having different wavelengths are incident on the base boards having different thicknesses. However, this optical pick-up device requires a plurality of light converging optical systems, to cause the complicated structure and higher cost, which is not preferable.
For this problem, there is developed an objective lens having a diffractive structure that can converge the light flux passing within the prescribed numerical aperture to spot light and that can make the light flux passing beyond the prescribed numerical aperture to be flared light. This objective lens can obtain spot light having a necessary diameter for the information recording or reproduction for the various optical information recording media such as, for example, CD and DVD.
Here, in the objective lens having the diffractive structure stated above, wave front aberration caused by the difference of the wavelength of the light source is generated originally, because the light flux having the numerical aperture smaller than the prescribed numerical aperture is used for the information recording or reproduction for the various optical information recording media such as CD and DVD commonly. Accordingly, it is necessary to make the characteristic of the wave front aberration to be better by some means. Further, there is request to increase the spot light amount for the optical information recording media having thick base board such as CD.
The object of the present invention is to provide the optical pick-up device and the objective lens which can control a phase shift of a wave front, in the objective lens which is used for the information recording or reproduction for the various optical information recording media such as CD and DVD commonly.
The object of the present invention is to provide the optical pick-up device and the objective lens which have the better characteristic of the wave front aberration, in the objective lens which is used for the information recording or reproduction for the various optical information recording media such as CD and DVD commonly.
Further, the object of the present invention is to provide the optical pick-up device and the objective lens which can increase the spot light amount, for the optical information recording medium such as CD having the thick base board, in the objective lens which is used for the information recording or reproduction for the various optical information recording media such as CD and DVD commonly.
An optical pick-up device has a first light source having wavelength xcex1, a second light source having wavelength xcex2 (xcex1 less than xcex2) and a converging optical system including an objective lens which converges the light fluxes emitted from the first light source and the second light source onto an information recording surface of the optical information recording medium, and being capable of performing the information recording and/or reproduction for the first optical information recording medium whose transparent base board thickness is t1, by using the first light source and the converging optical system, and being capable of performing the information recording and/or reproduction for tile second optical information recording medium whose transparent base board thickness is t2 (t1 less than t2), by using the second light source and the converging optical system, wherein when NA1 is a first necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the first optical information recording medium, using the first light source and tile converging optical system, and when NA2 (NA1 less than NA2) is a second necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the second optical information recording medium, using the second light source and at the converging optical system, at least one optical surface of the objective lens is provided with a central area which is from an optical axis to a prescribed distance corresponding to the second necessary numerical aperture NA2, and with a peripheral area which is outside the prescribed distance, and on the central area and the peripheral area, there are formed the diffractive structures having the diffractive ring-shaped zones blazed concerning the wavelength xcex0 (xcex1 less than xcex0 less than xcex2), and wherein when the information recording and/or reproduction is performed for the first optical information recording medium, controlled is the phase shift (phase difference) between the wave front of the light flux passing through the central area and the wave front of the light flux passing through the peripheral area. The present invention is explained as follows.
FIG. 1(a) is a schematic section in the optical axial direction of the objective lens, shown as a comparative example. FIG. 1(b) is a drawing showing illustratively the wave front aberration of the objective lens representing the comparative example, in the case of performing the information recording and/or reproduction for the first optical information recording medium. Since the phase shift equivalent to integer multiples of 2xcfx80 in the wave front aberration does not affect an image formation, the phase shift is excluded from the wave front aberration. The second necessary numerical aperture NA2 is 0.47 in this case. In the objective lens shown as the comparative example, since the peripheral area whose second necessary numerical aperture NA2 is equal to or greater than 0.47 is an exclusive area where the light flux performing the information recording and/or reproduction for the first optical information recording medium passes through, the blazing wavelength can be equal to the light wavelength in a practical use, accordingly, it is possible to keep the wave front aberration to be constant, in principle. However, in the central area whose necessary numerical aperture NA2 is equal to or less than 0.47, in order to obtain the adequate diffractive efficiency for both of the first and second optical information recording media, it is necessary to make the blazing wavelength for forming the diffractive ring-shaped zone to be different from the light wavelength in a practical use, and therefore, when the information recording and/or reproduction is performed for the first optical information recording medium, the phase shift of about 0.05 xcex1 is caused between the wave front of the light flux passing through the central area and the wave front of the light flux passing through the peripheral area.
In order to dissolve the phase shift, the central area and the peripheral area form the diffractive structures having blazed diffractive ring-shaped zones concerning the wave length xcex0 (xcexa less than xcex0 less than xcex2), in an objective lens of an optical pick-up device. Here, to blaze concerning the wavelength xcex0 means to design the diffractive ring-shaped zone so that diffractive efficiency may be raised to the highest level at the wavelength xcex0.
FIG. 2(a) is a schematic section in the optical axial direction of the objective lens of the optical pick-up device of the present invention. FIG. 2(b) is a drawing showing illustratively the wave front aberration of the objective lens of the optical pick-up device of the invention, in the case of performing the information recording and/or reproduction for the first optical information recording medium.
As understood from FIG. 2(b), it is possible to make the average value of the wave front aberration in the peripheral area to be equal to the average value of the wave front aberration in the central area practically, by forming the diffractive structures having the diffractive ring-shaped zones blazed concerning the wavelength xcex0 on both of the central area and the peripheral area, though the wave front aberration according to the shape of the ring-shaped zone is caused on the peripheral area. By setting the average values of the wave front aberrations to be equal each other mentioned above, the phase shift becomes 0 approximately or less than 0.05 xcex1, which can improve the wave front aberration.
In an optical pick-up device, there is a step generated in the optical axial direction, between an imaginary enveloping surface which is formed by jointing the crests in the direction of the optical axis of the diffractive ring-shaped zone of the central area and an imaginary enveloping surface which is formed by jointing the crests in the direction of the optical axis of the diffractive ring-shaped zone of the peripheral area, and the step is not less than 1 xcexcm and not grater than 10 xcexcm, therefore, when the information recording and/or reproduction is performed for the first optical information recording medium, for example, by providing the step which makes the light path difference to be equivalent to the integer multiples of the wave length xcex1, the characteristic of the change of the aberration for temperature changes can be improved without affecting the phase shift.
In an optical pick-up device, when the information recording and/or reproduction is performed for the first optical recording medium, it is preferable if the phase shift between the wave front of the light flux passing through the central area and the wave front of the light flux passing through the peripheral area is 0.05 xcex1 or less.
An optical pick-up device has a first light source having wavelength xcex1, a second light source having wavelength xcex2 (xcex1 less than xcex2) and a converging optical system including an objective lens which converges the light fluxes emitted from the first light source and the second light source onto an information recording surface of the optical information recording medium, and being capable of performing the information recording and/or reproduction for the first optical information recording medium whose transparent base board thickness is t1, by using the first light source and the converging optical system, and being capable of performing the information recording and/or reproduction for the second optical information recording medium whose transparent base board thickness if t2 (t1 less than t2), by using the second light source and the converging optical system, wherein when NA1 is a first necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the first optical information recording medium, using the first light source and the converging optical system, and when NA2 (NA1 less than NA2) is a second necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the second optical information recording medium, using the second light source and the converging optical system, at least one optical surface of the objective lens is provided with a central area which is from an optical axis to a prescribed distance corresponding to the second necessary numerical aperture NA2, and is provided with a peripheral area which is outside the prescribed distance, and on the central area, there is formed the central diffractive structure having the diffractive ring-shaped zone blazed concerning the light flux having the wavelength xcex0 (xcex1 less than xcex0 less than xcex2), and on the peripheral area, there is formed the peripheral diffractive structure having the diffractive ring-shaped zone blazed concerning the light flux having wavelength xcex0xe2x80x2 (xcex0xe2x80x2 less than xcex0), wherein the imaginary enveloping surface which is formed by jointing the crests directed to the optical axis of the diffractive ring-shaped zone of the central area is discontinuous with the imaginary enveloping surface which is formed by jointing the crests directed to the optical axis of the diffractive ring-shaped zone of the peripheral area, and wherein controlled is the phase shift between the wave front of the light flux passing through the central area and the wave front of the light flux passing through the peripheral area. The present invention is explained as follows.
FIG. 3(a) is a schematic section in the optical axial direction of the objective lens of the optical pick-up device of the invention. FIG. 3(b) is a drawing showing illustratively the wave front aberration of the objective lens of the optical pick-up device of the invention, in the case of performing the information recording and/or reproduction for the first optical information recording medium.
For the peripheral area whose second necessary numerical aperture NA2 is not less than 0.47, it is possible to keep the wave front aberration constant, by equalizing the blazing wavelength with the wavelength in use as shown in the upper part of FIG. 3(b). Further, for the central area whose second necessary numerical aperture NA2 is not greater than 0.47, it is possible to make the characteristic of the wave front aberration better, by bringing the average wave front closer to the average wave front in the peripheral area, (see the lower part in FIG. 3(b)), by making imaginary enveloping surface P1 which is formed by jointing the crests directed to the optical axis of the diffractive ring-shaped zone of the central area to be discontinuous with imaginary enveloping surface P2 which is formed by jointing the crests directed to the optical axis of the diffractive ring-shaped zone of the peripheral area.
In an optical pick-up device, there is a step generated in the optical axial direction, between an imaginary enveloping surface which is formed by jointing the crests in the direction of the optical axis of the diffractive ring-shaped zone of the central diffractive structure and an imaginary enveloping surface which is formed by jointing the crests in the direction of the optical axis of the diffractive ring-shaped zone of the peripheral diffractive structure, and the step is not less than 1 xcexcm and not greater than 10 xcexcm, therefore, when the information recording and/or reproduction is performed for the first optical information recording medium, for example, by providing the step which makes the light path difference to be equivalent to integer multiples of the wave length xcex1, the characteristic of the change of the aberration for temperature changes can be improved without affecting the phase shift.
In an optical pick-up device, when the information recording and/or reproduction is performed for the first optical recording medium, it is preferable if the phase shift between tile wave front of the light flux passing through the central area and the wave front of the light flux passing through the peripheral area is 0.05 xcex1 or less.
An optical pick-up device has a first light source having wavelength xcex1, a second light source having wavelength xcex2 (xcex1 less than xcex2) and a converging optical system including an objective lens which converges the light fluxes emitted from the first light source and the second light source onto an information recording surface of the optical information recording medium, and being capable of performing the information recording and/or reproduction for the first optical information recording medium whose transparent base board thickness is t1, by using the first light source and the converging optical system, and being capable of performing the information recording and/or reproduction for the second optical information recording medium whose transparent base board thickness is t2 (t1 less than t2), by using the second light source and the converging optical system, wherein when NA1 is a first necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the first optical information recording medium, using the first light source and the converging optical system, and when NA2 (NA1 less than NA2) is a second necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the second optical information recording medium, using the second light source and the converging optical system, at least one optical surface of the objective lens is provided with a central area which is from an optical axis to a prescribed distance corresponding to the second necessary numerical aperture NA2, and is provided with a peripheral area which is outside the prescribed distance, and on the central area, there is formed the central diffractive structure having the diffractive ring-shaped zone blazed concerning the light flux having the wavelength xcex0 (xcex1 less than xcex0 less than xcex2), and wherein lightened is at least either one of the wave front aberration generated by the difference between the light source wavelength xcex1 and the blazing wavelength xcex0, when the information recording and/or reproduction is performed for the first optical information recording medium, and the wave front aberration generated by the difference between the light source wavelength xcex2 and the blazing wavelength xcex0, when the information recording and/or reproduction is performed for the second optical information recording medium. The invention will be explained as follows.
The invention is attained in order to improve the minute wave front aberration, that is generated by the difference of the blazing wavelength and the wavelength in use in each of the diffractive ring-shaped zone. If there is the wave front aberration of low order power of the aperture in the objective lens for the optical pick-up, it is generally said that the characteristics of a jitter signal and others become worse. Accordingly, when the wave front aberration generated by the difference between the blazing wavelength and the wavelength in use is expressed by the power series of NA, it is desirable to design the third-order spherical aberration and the fifth-order spherical aberration which have been corrected beforehand. A designing technique which controls the third-order or higher order spherical aberrations concretely is the incorporation of the low-order wave front aberration in advance, which is generated by the difference between the blazing wavelength and the wavelength in use, into the design, when the pitch of the ring-shaped zone and the macroscopic aspheric surface are designed. Especially, the diffractive ring-shaped zone of the central diffractive structure including the optical axis has a large area, where the low-order spherical aberration is easily generated, which must be considered as the important matter.
An optical pick-up device has a first light source having wavelength xcex1, a second light source having wavelength xcex2 (xcex1 less than xcex2) and a converging optical system including an objective lens which converges the light fluxes emitted form the first light source and the second light source onto an information recording surface of the optical information recording medium, and being capable of performing the information recording and/or reproduction for the first optical information recording medium whose transparent base board thickness is t1, by using the first light source and the converging optical system, and being capable of performing the information recording and/or reproduction for the second optical information recording medium whose transparent base board thickness is t2 (t1 less than t2), by using the second light source and the converging optical system, wherein the NA1 is a first necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the first optical information recording medium, using the first light source and the converging optical system, and when NA2 (NA1 less than NA2) is a second necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the second optical information recording medium, using the second light source and the converging optical system, at least one optical surface of the objective lens is provided with a central area which is from an optical axis to a prescribed distance corresponding to the second necessary numerical aperture NA2, and is provided with a peripheral area which is outside the prescribed distance, and on the central area and the peripheral area, there are formed the diffractive structures provided with the diffractive ring-shaped zones, and wherein the diffractive structure of the central area and at least the diffractive structure of the peripheral area contacting the central area are blazed concerning the wavelength xcex0 (xcex1 less than xcex0 less than xcex2). The invention will be explained as follows.
When the recording and/or reproduction is performed at high speed for the second optical information recording medium such as CD, it is necessary to converge a large amount of light by the objective lens. However, the increase of the numerical aperture NA (NA2 in this case) of the area (the central area in this case) where the aberration is to be corrected causes the too small beam and the bad influence such as the deterioration of the tolerance. Accordingly it is required to increase spot light amount, without increasing the numerical aperture NA of the area where the aberration is to be corrected. As a result of the investigation by the inventers for the present invention, it is understood that the area (the peripheral area in this case) which is not used for the information recording and/or reproduction for the second optical information recording medium, such as the area where the aberration is not corrected and the flare is generated, can contribute to the light amount of the spot for the information recording and/or reproduction for the second optical information recording medium. Based on the result of the investigation, by blazing the diffractive structure in the central area and at least the diffractive structure of the peripheral area contacting the central area, concerning the light flux having wavelength xcex0, the invention can obtain the large light amount of the spot, to perform the information recording and/or reproduction for the second optical information recording medium.
Here, by equalizing the blazing wavelength xcex0 of the peripheral area to the wavelength xcex1 used for the first optical information recording medium, it is possible to obtain the higher diffractive efficiency of the diffractive structure in the peripheral area for the first optical information recording medium, however, the diffractive efficiency becomes lower for the wavelength xcex2 used for the second optical information recording medium. Therefore, the blazing wavelength of the diffractive structure in the area (near the border to the central area in the peripheral area, that is, the area contacting the central area) whose numerical aperture NA is relatively small, for example, NA is 0.47 to 0.52 in the peripheral area, is equalized to the blazing wavelength of the diffractive structure in the central area, then, the diffractive efficiency concerning the light flux having wavelength xcex2 becomes large for this area, and it is possible to obtain large amount of light flux for the information recording and/or reproduction for the second optical information recording medium.
An objective lens of an optical pick-up device has a first light source having wavelength xcex1, a second light source having wavelength xcex2 (xcex1 less than xcex2) and a converging optical system including an objective lens which converges the light fluxes emitted from the first light source and the second light source onto an information recording surface of the optical information recording medium, and being capable of performing the information recording and/or reproduction for the first optical information recording medium whose transparent base board thickness is t1, by using the first light source and the converging optical system, and being capable of performing the information recording and/or reproduction for the second optical information recording medium whose transparent base board thickness is t2 (t1 less than t2), by using the second light source and the converging optical system, wherein when NA1 is a first necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the first optical information recording medium, using the first light source and the converging optical system, and when NA2 (NA1 less than NA2) is a second necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the second optical information recording medium, using the second light source and the converging optical system, at least one optical surface of the objective lens is provided with a central area which is from an optical axis to a prescribed distance corresponding to the second necessary numerical aperture NA2, and is provided with a peripheral area which is outside the prescribed distance, and on the central area and the peripheral area, there are formed the diffractive structures having the diffractive ring-shaped zones blazed concerning the wavelength xcex0 (xcex1 less than xcex0 less than xcex2), and wherein when the information recording and/or reproduction is performed for the first optical information recording medium, controlled is the phase shift between the wave front of the light flux passing through the central area and the wave front of the light flux passing through the peripheral area.
In an objective lens of an optical pick-up device, there is a step generated in the optical axial direction, between an imaginary enveloping surface which is formed by jointing the crests directed to the optical axis of the diffractive ring-shaped zone of the central area and an imaginary enveloping surface which is formed by jointing the crests directed to the optical axis of the diffractive ring-shaped zone of the peripheral area, and the step is not less than 1 xcexcm and not greater than 10 xcexcm.
In an objective lens of an optical pick-up device, when the information recording and/or reproduction is performed for the first optical recording medium, the phase shift between the wave front of the light flux passing through the central area and the wave front of the light flux passing through the peripheral area is 0.05 xcex1 or less.
An objective lens of an optical pick-up device has a first light source having wavelength xcex1, a second light source having wavelength xcex2 (xcex1 less than xcex2) and a converging optical system including an objective lens which converges the light fluxes emitted from the first light source and the second light source onto an information recording surface of the optical information recording medium, and being capable of performing the information recording and/or reproduction for the first optical information recording medium whose transparent base board thickness is t1, by using the first light source and the converging optical system, and being capable of performing the information recording and/or reproduction for the second optical information recording medium whose transparent base board thickness is t2 (t1 less than t2), by using the second light source and the converging optical system, wherein when NA1 is a first necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the first optical information recording medium, using the first light source and the converging optical system, and when NA2 (NA1 less than NA2) is a second necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the second optical information recording medium, using the second light source and the converging optical system, at least one optical surface of the objective lens is provided with a central area which is from an optical axis to a prescribed distance corresponding to the second necessary numerical aperture NA2, and is provided with a peripheral area which is outside the prescribed distance, and on the central area, there is formed the central diffractive structure having the diffractive ring-shaped zone blazed concerning the light flux having the wavelength xcex0 (xcex1 less than xcex0 less than xcex2), and on the peripheral area, there is formed the peripheral diffractive structure having the diffractive ring-shaped zone blazed concerning the light flux having wavelength xcex0xe2x80x2 (xcex0xe2x80x2 less than xcex0), wherein the imaginary enveloping surface which is formed by jointing the crests directed to the optical axis of the diffractive ring-shaped zone of the central area is discontinuous with the imaginary enveloping surface which is formed by jointing the crests directed to the optical axis of the diffractive ring-shaped zone of the peripheral area, and where in controlled is the phase shift between the wave front of the light flux passing through the central area and the wave front of the light flux passing through the peripheral area.
In an objective lens of an optical pick-up device, there is a step generated in the optical axial direction, between an imaginary enveloping surface which is formed by jointing the crests directed to the optical axis of the diffractive ring-shaped zone of the central diffractive structure and an imaginary enveloping surface which is formed by jointing the crests directed to the optical axis of the diffractive ring-shaped zone of the peripheral diffractive structure, and the step is not less than 1 xcexcm and not greater than 10 xcexcm.
In an objective lens of an optical pick-up device, when the information recording and/or reproduction is performed for the first optical recording medium, the phase shift between the wave front of the light flux passing through the central area and the wave front of the light flux passing through the peripheral area is 0.05 xcex1 or less.
An objective lens of an optical pick-up device has a first light source having wavelength xcex1, a second light source having wavelength xcex2 (xcex1 xcex2) and a converging optical system including an objective lens which converges the light fluxes emitted from the first light source and the second light source onto an information recording surface of the optical information recording medium, and being capable of performing the information recording and/or reproduction for the first optical information recording medium whose transparent base board thickness is t1, by using the first light source and the converging optical system, and being capable of performing the information recording and/or reproduction for the second optical information recording medium whose transparent base board thickness is t2 (t1 less than t2), by using the second light source and the converging optical system, wherein when NA1 is a first necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the first optical information recording medium, using the first light source and the converging optical system, and when NA2 (NA1 less than NA2) is a second necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the second optical information recording medium, using the second light source and the converging optical system, at least one optical surface of the objective lens is provided with a central area which is from an optical axis to a prescribed distance corresponding to the second necessary numerical aperture NA2, and is provided with a peripheral area which is outside the prescribed distance, and on the central area, there is formed the central diffractive structure having the diffractive ring-shaped zone blazed concerning the light flux having the wavelength xcex0 (xcex1 less than xcex0 less than xcex2), and wherein lightened is at least either one of the wave front aberration generated by the difference between the light source wavelength xcex1 and the blazing wavelength xcex0, when the information recording and/or reproduction is performed for the first optical information recording medium, and the wave front aberration generated by the difference between the light source wavelength xcex2 and the blazing wavelength xcex0, when the information recording and/or reproduction is performed for the second optical information recording medium.
An objective lens of an optical pick-up device has a first light source having wavelength xcex1, a second light source having wavelength xcex2 (xcex1 less than xcex2) and a converging optical system including an objective lens which converges the light fluxes emitted from the First light source and the second light source onto an information recording surface of the optical information recording medium, and being capable of performing the information recording and/or reproduction for the first optical information recording medium whose transparent base board thickness is t1, by using the first light source and the converging optical system, and being capable of performing the information recording and/or reproduction for the second optical information recording medium whose transparent base board thickness is t2 (t1 less than t2), by using the second light source and the converging optical system, wherein when NA1 is a first necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the first optical information recording medium, using the first light source and the converging optical system, and when NA2 (NA1 less than NA2) is a second necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the second optical information recording medium, using the second light source and the converging optical system, at least one optical surface of the objective lens is provided with a central area which is from an optical axis to a prescribed distance corresponding to the second necessary numerical aperture NA2, and is provided with a peripheral area which is outside the prescribed distance, and on the central area and the peripheral area, there are formed the diffractive structures provided with the diffractive ring-shaped zones, and wherein the diffractive structure of the central area and the diffractive structure positioned at least contacting with the central area are blazed concerning the wavelength xcex0 (xcex1 less than xcex0 less than xcex2).
The xe2x80x9cdiffractive structurexe2x80x9d used in the present specification means the section which is provided with relief on the surface of the objective lens and has the function to converge or diverge the light flux by diffraction. Concerning the form of relief, as shown in FIG. 2(a), for example, there is known the shape wherein ring-shaped zones are formed to be the concentric circles around the optical axis representing the center, and when its section on a plane including an optical axis is viewed, each ring-shaped zone is like the saw-tooth. The form of relief includes above-mentioned shape, and this shape is named xe2x80x9cthe diffractive ring-shaped zonexe2x80x9d especially.
In the present specification, in a narrow sense, the objective lens means the lens having light converging function and being arranged to face the optical information recording medium at the closest position to the optical information recording medium, under the condition that the optical information recording medium is installed on the optical pick-up device. In a wide sense, the objective lens means the lens group which can be moved by an actuator together with the lens at least in the direction of the optical axis of the lens. Here, such lens group means at least one or more lens pieces (for example, two pieces). Accordingly, in the present specification, the numerical aperture NA of the objective lens on the optical information recording medium side (an image side) means the numerical aperture NA of the objective lens through which the light flux pass the lens surface positioned to be closest to the optical information recording medium. Further, in the present specification, the necessary numerical aperture NA means the numerical aperture which is regulated by the standard of each optical information recording medium, or means the numerical aperture of the objective lens having the diffractive limit power which can obtain the spot diameter necessary to perform recording or reproduction of information for each of the optical information recording media, in accordance with the wavelength of the light source to be used.
In the present specification, the second optical information recording medium means the optical disks of various CD groups representing, for example, CD-R, CD-RW, CD-Video and CD-ROM, while the first optical information recording medium means the optical disks of various DVD groups representing DVD-ROM, DVD-RAM, DVD-R, DVD-RW and DVD-Video. Still further, in the present specification, when the thickness t of the transparent base board is mentioned, it includes the case of t=0.
In this specification, the information recording and/or reproduction means to record information on the information recording surface of the optical information recording medium as mentioned above, and/or to reproduce information recorded on the information recording surface. The optical pick-up device of the invention can be used for either the recording only or the reproduction only, or for both of the recording and the reproduction. Further, the optical pick-up device of the invention can be used for either the recording for one information recording medium and the reproduction for the other information recording medium, or for the recording or the reproduction for one information recording medium and for the recording and reproduction for the other information recording medium. Incidentally, the reproduction in this case includes only reading of information.